Is the picture of the monster/ monster group at the beginning of a battle the most powerful one of the group?

I assume you are talking about MM1 or MM2? Monsters are displayed differently in the later games.

Anyway, I don't think the pictured monster is necessarily the most powerful, just that it is the "leader" of that pack of enemies. This is often the most powerful monster, but not always. At least, that's what I remember; someone please correct me if I'm wrong.